- Before replacing the data, select the text box on your label where you want the merged data to appear. If no text box is selected, the new data will be merged into a new text box instead.

- Then go back to the left-hand side and select Import Data
- Select Replace Spreadsheet

- Finally, select Browse for File to import your new Spreadsheet. Follow the steps to Choose Fields then Arrange Fields to Finish.
To learn more about this, watch this video: